Film stock is the term for photographic film on which motion pictures are shot. There are several variables in classifying stocks; in practice, one orders raw stock by a code number, based on desired sensitivity to light.
A piece of film consists of a light-sensitive emulsion applied to a tough, transparent base, attached to anti-halation backing. Originally the highly flammable cellulose nitrate, was used. In the 1930s, film manufacturers introduced "safety film" with a cellulose triacetate plastic base. All amateur film stocks were safety film, but the use of nitrate persisted for professional releases. Kodak discontinued the manufacture of nitrate base in 1951, and the industry transitioned entirely to safety film in 1951 in the United States and by 1955 internationally. Since the 1980s a growing number of films have used polyester film stock.
The emulsion is comprised of silver halide grains suspended in a gelatin colloid; in the case of color film, there are three layers of silver halide which are mixed with color couplers and interlayers which filter specific light spectra. These end up creating yellow, cyan, and magenta layers in the negative after development.
Development chemistry may produce either a positive or negative image. Camera films that produce a positive image are known as reversal films; processed film of this type can typically be directly projected. Negative films require additional transferring; since negative films are much more commonly used, terms are based on the steps needed to produce a viewable finished print: one speaks of negatives and positives.
Different emulsions and development processes exist for a variety of image recording possibilities: the two most common of which are black and white and color. However, there are also variant types, such as infrared film (in black and white or color); specialist technical films, such as those used for X-rays; and obsolete processes, such as orthochromatic film. Generally, however, the vast majority of stock used today is "normal" (visible spectrum) color, although "normal" black and white also commands a significant minority percentage.
Film is also classified according to its gauge and the arrangement of its perforations— gauges range from 8 mm to 70 mm or more, while perforations may vary in shape, pitch, and positioning. The film is also distinguished by how it is wound with regard to perforations and base or emulsion side, as well as whether it is packaged around a core, a daylight spool, or within a cartridge. Depending on the manufacturing processes and camera equipment, lengths can vary anywhere from 25 to 2000 feet. Common lengths include 25 feet for 8 mm, 50 feet for Super 8, 100 and 400 feet for 16 mm, 400 and 1000 feet for 35 mm, and 1000 for 65/70 mm.
A critical property of a stock is its film speed, determined by ASA or its sensitivity to light listed by a measurement on the raw stock which must be chosen with care. Speed determines the range of lighting conditions under which the film can be shot, and is related to granularity and contrast, which influence the look of the image.
Another important quality of color film stock in particular is its color balance, which is defined by the color temperature at which it accurately records white. Tungsten lighting is defined at 3200 K, which is considered "warmer" in tone and shifted towards orange; daylight is defined at 5500 K, which is considered "colder" and shifted towards blue. This means that unfiltered tungsten stock will look normal shot under tungsten lights, but blue if shot during daylight. Obversely, daylight stock shot in daylight will look normal, but orange if shot under tungsten lights. Color temperature issues such as these can be compensated for by other factors such as lens filters and color gels placed in front of the lights. The color temperature of a film stock is generally indicated next to the film speed number - e.g. 500T stock is color film stock with an ASA of 500 and balanced for tungsten light; 250D would have an ASA of 250 and be balanced for daylight.
Motion picture film is known to be a highly unstable medium: improperly preserved film can deteriorate in a period of time much faster than many photographs or other visual presentations. Owners of home-made films often find that their film can become brittle and unwatchable in the space of a few years. Decaying film stock gives off an odor similar to that of vinegar, which is why film buffs often refer to such decaying as "vinegar syndrome."
The distinction between camera stocks and print stocks involves a difference in the recording process. Typically, the original camera negative (OCN) is spliced by a negative cutter who assembles it for an answer print. When the answer print has been approved, interpositive (IP) prints are struck from it, and each IP is then used to make several internegative (IN) copies. The release prints are then generated from the INs. Recently, with the development of digital intermediate (DI), it has become possible to completely edit, composite visual effects, and color grade the image digitally at full resolution and bit-depth. In this workflow, the answer print is generated digitally and then written out to the IP stage using a laser film printer.
Due to the specialized nature of the exposure and the higher degree of control afforded by the film lab equipment, these intermediate and release stocks are specially designed solely for these applications and are generally not feasible for camera shooting. Because intermediates only function to maintain the image information accurately across duplication, each manufacturer tends to only produce one or two different intermediate stocks. Similarly, release print stocks usually are available only in two varieties: a "normal" print or a deluxe print with greater latitude and contrast.
Lately it has become possible to transfer video images, including films scanned at high resolution, back to film stocks by making a digital intermediate, which can then be recorded out to a film interpositive with a laser film printer. The first major live-action film to use this process entirely was O Brother Where Art Thou. Prior to this, the video master was transferred from tape to film through one of several methods: CRT recorder, laser film printer, Kinescope, or electron beam recorder (EBR). Theater performances have been preserved with Kinescope for many years—the 1964 New York production of Hamlet with Richard Burton, for example, was shot on video and printed as a film that was released in movie theaters using this process. Digital video equipment has made this approach easier; theatrical-release documentaries and features originated on video are now being produced this way.